FIG. 9(a), for example, is a perspective view illustrating a schematic construction of a conventional semiconductor optical modulator described in fifty-third Applied Physics Society, Scientific Lecture Meeting Proceedings 16a-x-1 (p. 911) 1992 autumn. In the figure, reference numeral 1 designates a p side electrode, reference numeral 2 designates a p type InP cladding layer, reference numeral 4 designates an n type InP cladding layer, reference numeral 5 designates an n type InP substrate, reference numeral 6 designates an n side electrode, reference numeral 3 designates an active region of a multi quantum well structure formed on a ridge portion 11, and reference numeral 12 designates a CW (continuous wave) laser light incident on the active region 3, having a wavelength of 1.585 .mu.m. Reference numeral 12' designates modulated light irradiated from the active region 3.
FIG. 9(b) is a partially enlarged view illustrating a layer structure of the active region 3. In the figure, reference numeral 16 designates an InP barrier layer 11 nm thick, reference numeral 17 designates an In.sub.0.49 Ga.sub.0.51 As well layer having a lattice strain of -0.3%, and 11.5 nm thick. The active region 3 comprises 20 periods of a repetition of In.sub.0.49 Ga.sub.0.51 As well layer 17 and InP barrier layer 16, forming a multi quantum well structure including tensile stress.
A description is given of the operation.
When a laser light 12 is incident on the front surface 3' of the active region 3 in the ridge portion 11 as shown in FIG. 9(a), and when no electric field is applied to the active region 10, the laser light 12 is transmitted in a waveguide comprising the active region 10 and is emitted from the rear facet 3".
When a negative voltage is applied to the p side electrode 1 and a positive voltage is applied to the n side electrode 6, and this reverse bias voltage forms an electric field applied to the active region 3, the active region 3 operating as a waveguide functions as an absorption layer due to the quantum confining Stark effect and absorbs the laser light 12 and the advance of the laser light 12 is cut off.
The laser light 12 incident on the active region 3 has a plane of polarization as an electromagnetic wave. In a case where the light source emitting the laser light 12 is a white light laser, the laser light 12 has two polarization planes, TE mode and TM mode. In a semiconductor optical modulator having a general structure, since the light is absorbed mainly in either TE mode or TM mode, even when an electric field is applied to the active region to make it function as an absorption layer, the light of the other mode might be transmitted. In a structure in which the laser light 12 advances in parallel to the junction plane of the active region as shown in FIG. 9(a), the light of TE mode is mainly absorbed, and the light of TM mode is transmitted without being absorbed even if the reverse bias voltage is increased to intensify the electric field, whereby the transmitted light of TM mode reaches the light receiving element, thereby producing an erroneous operation of the light receiving element.
The active region 3 of the semiconductor optical modulator shown in FIG. 9(a), is shown in FIG. 9(b) in an enlarged view, and comprises 20 periods of a repetition of In.sub.0.49 Ga.sub.0.51 As well layer 17 having a lattice strain of -0.3% and InP barrier layer 16, forming a multi quantum well structure. In this way, by introducing a tensile strain of 0.3% into the active region 3, incident laser light 12 can be absorbed without dependency on the plane of polarization.
Since the electric field dependency of transmissivity in the active region 3 including a tensile stress of 0.3% is as shown in FIG. 8, when a reverse bias voltage above 3 V is applied between p side electrode 1 and n side electrode 6 of the semiconductor optical modulator having the active region 3, the absorption in the TE mode and the absorption in the TM mode of the laser light 12 of a wavelength 1.585 .mu.m are approximately equal to each other, thereby reducing the dependency on the plane of polarization.
The absorption/transmission operation is repeated at a high speed in accordance with on/off state of the reverse bias voltage of above 3 V of a modulation signal applied between the p side electrode 1 and the n side electrode 6, whereby the Cw laser light 12 is emitted as a pulsed modulated light 12' in accordance with the modulation signal from the rear face 3" of the active region 3.
The prior art semiconductor optical modulator is constituted as described above, and the polarization plane dependency can be reduced to some extent, but the absorption coefficients of both TM and TE modes cannot be increased unless a reverse bias voltage above 3 V is applied. Further, when the number of repetitions forming the multi quantum well is further increased from 20 periods of a repetition of In.sub.0.49 Ga.sub.0.51 As well layer 17 and InP barrier layer 16 to increase the light extinction ratio, the negative lattice strain also increases at the same time, and the crystal would be fractured by the stress due to the increased strain. Further, it is impossible to make a two dimensional array which modulates a plurality of incident light beam and outputs a plurality of modulated light beam as a semiconductor optical modulator.